1. Technical Field
The present invention relates to an image forming apparatus and an image forming method using the apparatus, which is provided with a transfer roller transferring an image formed on an image carrier to a recording medium.
2. Related Art
There is disclosed an image forming apparatus which develops an electrostatic latent image to be visible using a liquid developer with high viscosity in which a toner containing solid content is dispersed into a carrier liquid (for example, refer to JP-A-2009-14808, FIG. 1). In the image forming apparatus, four image forming stations are provided which form images with different colors from each other, and toner images formed by the respect image forming stations are overlapped on an intermediate transfer belt so as to form a color image. The intermediate transfer belt is suspended on a pair of a driving roller and a driven roller which are disposed separately from each other, and drives the driving roller by a belt driving motor so as to move in a predetermined direction while the color image is carried. In addition, a secondary transfer apparatus which is commercially available in a dry image forming apparatus is provided to face the driving roller, the color image is secondarily transferred onto a recording medium such as paper, film or cloth. In other words, in the secondary transfer position, a secondary transfer roller of which an external peripheral surface is covered by an elastic member such as rubber is disposed to face the driving roller with the intermediate transfer belt interposed therebetween, so that a nip is formed between the intermediate transfer belt and the elastic member. Then, by driving the driving roller by a belt driving motor, the intermediate transfer belt is rotated and the secondary transfer roller is rotatably driven with respect to the intermediate transfer belt, and the recording medium is nipped and transported between the nip. During nipping and transporting the recording medium, a color image on the intermediate transfer belt is secondarily transferred onto the recording medium.
Since the apparatus described in JP-A-2009-14808 (FIG. 1) employs a wet scheme using liquid developer, the recording medium passed through the nip is not separated from the intermediate transfer belt, but transported toward the image forming stations in a state of being attached to the intermediate transfer belt, so that so-called paper jam occurs. As one of measures for solving the problem, there is proposed a transfer technique which is used in a stencil printing apparatus. The reason is as follows.
In the stencil printing apparatus, an impression cylinder is provided which is provided with a paper gripper to hold the tip end of paper, and rotates by a driving force applied from a driving unit such as a motor. The impression cylinder as configured above rotates while gripping the tip end of paper by the paper gripper, and pushes the paper with respect to the impression cylinder which winds a proofed master, so that printing is performed by ink (for example, refer to JP-A-2000-238400, FIG. 1). For this reason, in spite of using or not high-viscosity ink is used, the printed paper is not attached to the cylinder, but effectively separated from the cylinder on the downstream side of an attached position. Then, it may be considered that the secondary transfer roller is configured as the impression cylinder as described above, that is, a grip unit may be provided to grip the recording medium in the secondary transfer roller. That is, while the tip end (non-image portion) of the recording medium is gripped by the grip unit, the secondary transfer roller rotates, so that the recording medium passed through the nip can be effectively separated from the intermediate transfer belt, and paper jams can be effectively prevented.
However, when the transfer technique described in JP-A-2000-238400 (FIG. 1) is employed to the image forming apparatus, there may occur a problem such as the degradation in the image quality or the contamination of the rear surface of the recording medium. That is, in the image forming apparatus, an elastic layer such as rubber or resin is formed on an outer peripheral surface of the secondary transfer roller in order to form a nip, and moreover a load (for example, 50 to 100 kgf in case of the image forming apparatus in liquid developing scheme) is applied on the surface of the image carrier such as the intermediate transfer belt or the intermediate transfer drum from the secondary transfer roller. Therefore, when the secondary transfer roller stops in a state in which the elastic layer of the secondary transfer roller comes into contact with the surface of the image carrier so as to form the nip, the elastic layer is deformed while in the stopping state. Then, when image forming is performed using the secondary transfer roller which has been deformed, transfer defect or speed variation occurs in a corresponding image forming process and, as a result, the image defect such as color deviation or banding of the image occurs.
In addition, there is a need to move the secondary transfer roller to a predetermined position (recording medium gripping position) in order to grip the recording medium by the grip unit, but in the case where the transfer technique described in JP-A-2000-238400 (FIG. 1) is employed, there is a need to position the secondary transfer roller on the predetermined position by freely running the secondary transfer roller in a state in which the recording medium is not gripped. At this time, in a case where there is a liquid developer on the surface of the image carrier, since the liquid developer is attached to the outer peripheral surface of the secondary transfer roller running freely, the liquid developer is attached to the rear surface of the recording medium when the recording medium passes through the nip, so that he recording medium may be contaminated.